1. Field of the Invention
The present invention relates to a screen data transmitting device for compressing image data to be displayed on a screen of a computer and transmitting it via a network to any other information appliances.
2. Description of the Related Art
Virtual network computing (VNC) system has been used widely which transmits in real time, through a network, image data to be displayed on a screen of a personal computer (PC) so that the image data may be displayed on a remote screen display terminal and remotely manipulated from the PC. Further, such a system has also been used widely which transmits in real time such image data as data of a moving image to be displayed on a PC screen to a screen display terminal, for example, a projector by using a wireless LAN such as IEEE802.11b or IEEE802.11g so that the data may be displayed there.
Both of these systems are based on a technology for acquiring and compressing a PC image in real time and transmitting it to any other information appliances via a network. Conventionally, such real time transmission of screen image data has involves periodic acquisition and compression of image data of an entire screen or acquisition and compression of image data of only a changed portion of a screen. The image data has been compressed either by a one-time compression scheme or by such a scheme as described in Jpn. Pat. Appln. KOKAI Publication No. 10-126773 for attempting to compress image data a plurality of number of times for each frame and employing a higher compression ratio.
Jpn. Pat. Appln. KOKAI Publication No. 2004-86550 is also available which divides a screen into regions that respectively correspond to a plurality of application programs for drawing images so that a compression scheme may be set to each of the regions.
On a display screen of a PC etc., a variety of types of images are mixed such as a natural image, a character image, and a CG image. For example, in a case where only such a compression scheme as used in the JPEG standards is used for cutting off a high-frequency component of an image, a natural image can be compressed at a high compression ratio but a character image in a monochromatic background cannot be compressed at a high compression ratio in condition where its high image quality is maintained, as compared to the case of run length coding. Conversely, in the case of using only run length coding, a character image in a monochromatic background can be compressed distortion-free at a high compression ratio but a natural image with finely changing colors cannot be compressed at high ratios. Therefore, it is difficult to always obtain a high compression ratio using a single compression scheme.
Further, a compressed image data display region for which one of recent application programs is held responsible includes a plurality of components such as a character, a natural image, and an icon, which contain images having different properties in many cases.
Conventionally, the method for employing only one compression always and the method for setting a compression scheme for each of application programs have suffered from a problem that a high compression ratio cannot be obtained because an appropriate compression scheme cannot be set in such a manner as to accommodate properties of each of regions of a screen.
Further, as described in Jpn. Pat. Appln. KOKAI Publication No. 10-126773, according to a method for comparing a plurality of compression schemes always, it is possible to select and utilize an appropriate one of the compression schemes according to properties of an image to enhance a compression ratio but, to determine any one of the compression schemes, it is necessary to compress a transmit image by a plurality of the compression schemes, thus resulting in a problem of taking much of time in compression processing or requiring a dedicated processing apparatus.
On the other hand, such a method as described in Jpn. Pat. Appln. KOKAI Publication No. 2004-86550 for setting a compression scheme for each application program finds it difficult to select an appropriate one of the schemes because a display region correlated with one of the application programs includes images with different properties in many cases. Moreover, even with the same application, a design or contents to be displayed vary with user settings and operating condition, thus resulting in a problem that compression cannot always be performed properly with a compression scheme correlated fixedly.